Properties of three different molecular forms of the alpha 2 plasmin inhibitor.

Abstract

The α2plasmin inhibitor prepared by affinity chromatography on plasminogen-Sepharose 4B appears mainly as a molecular form with Mr67000 but also as forms with Mr65000 and 60000. The 67000-Mr and 60000. The 67000-Mrform spontaneously converts to the lower-molecular-mass forms. The rate of conversion is temperature-dependent. All three molecules react with specific immuno globulin against α2plasmin inhibitor. The plasminogen-binding capacity of the 67000-Mr form is lost upon its conversion into the 65000-Mrform, but it has retained its capacity to inhibit plasmin. Upon conversion to the 60000-Mr form the capacity to inhibit plasmin is also lost. The presumed catalytic effect of a copurified serine proteinase on these reactions could not be substantiated since the addition of a number of known serine proteinases and serine proteinase inhibitors did not influence the rate of conversion of the 67000-Mrform to the modified forms. The functional properties of the 65000-Mrnon-plasminogen-binding α2plasmin inhibitor form were compared to those of the 67000-Mrplasminogen-binding form. In a clot lysis assay the plasminogen-binding inhibitor was a more efficient inhibitor of fibrinolysis than the non-plasminogen-binding inhibitor. Plasminogen was displaced from fibrin more readily by the plasminogen-binding inhibitor than by the non-plasminogen-binding form, and activation of Glu-plasminogen by urokinase and tissue activator was more rapid with the plasminogen-binding inhibitor than with the non-plasminogen-binding form. Kinetic studies showed that the plasminogen-binding α2plasmin inhibitor reacted more readily with plasmin than the non-plasminogen-binding form and that the inactivation of plasmin by the former was a biphasic process in the absence of 6-aminohexanoic acid, but monophasic in its presence. Inactivation of plasmin by the non-plasminogen-binding α2plasmin inhibitor was a monophasic reaction both in the absence and the presence of 6-aminohexanoic acid and identical to the reaction with the plasminogen-binding form in the presence of 6-aminohexanoic acid.